Heat-treating solids



Aug. 26, 1952 P. H. RoYs-rR 2,603,481

HEAT-TREATING SOLIDS Filed Sept. 26, 1947 Y a Sheets- Sheet 1 77/ERMALBALANCE TEMPERA TURE F THERMAL WWW/58552014! Q 3 8 zgnz Aug. 26, 1952 P.H. ROYSTER HEAT-TREATING SOLIDS Filed Sept. 26, 1947 TEMPERA 77/95 FTVEPMA L OVEPBA LANCE OVEPBA LANCE 5 Sheets-Sheet 2 m BY i p PM $4M ZT952 P. H. ROYSTER 2,608,481

HEAT-TREATING SOLIDS Filed Sept. 26, 1947 Y 3 Sheets-Sheet 5 THERMALUNDER 7 EMPE/PA TUE'E Patented Aug. 26, 1952 HEAT-TREATING SOLIDS PercyH. Royster, Chevy Chase, Md., assignor to Pickands Mather & 00.,Cleveland, Ohio, a copartnership Application September 26, 1947, SerialNo. 776,351

This invention relates to the art of working up aggregates of finelydivided iron ore concentrates, and is concerned more specifically withthe step of indurating raw pellets formed of moist oxidic iron particlesresulting as product from beneficiating a low grade oxidic iron orematerial such, for example, as the ferruginous cherts known in Minnesotaas taconites. The present applica tion contains subject matter in commonwith my application Serial No. 605,861, filed July 19, 1945, now PatentNo. 2,533,142, and is to be considered as a continuation-in-partthereof.

Beneficiation methods involve more or less fine grinding of the lowgrade iron ore material, with the result that the beneficiated productis in a state of subdivision too fine to be directly usable in the blastfurnace and hence requires the practice of some type of agglomeratingoperation whereby the fine particles may be formed into masses of a sizeand structure usable in'the blast furnace. One operable mode of soagglomerating iron oxide particles is to mold the finely dividedmaterial into pellets (or small spheroidal masses) by subjecting them'toa rolling action while somewhat moist. The disclosure of U. S. PatentNo. 2,411,873 to' Charles V; Firth is here referred to for a fulldisclosure of one operable mode of carrying out a pelletizing step.

When the pellets have been formed in optimum size (viz., one-fourth inchto 1.5 inches diameter) from particles having a desired sizedistribution, and with the moisture content controlled-between 5 and 15%by weight-to the optimum amount for the particular ore and particularparticle size distribution, "they enjoy in freshly formed (moist) statea certain degree of mechan-- ical strength. Thus, they may be droppedthree times from a height of 6 inches onto a steel plate without seriousfracture or deformation: when piled carefully on themselves they cansupport an 18-32 inch layer of themselves without the lowest layer ofpellets being crushed under the superimposed gravitational burden.

However, even if this degree of mechanical strength could be retainedupon drying out, it would not be suificient to permit shipping the rawpellets in mass or any other-handling usual with ores per se. It isessential to indurate the raw pellets before they are industriallyacceptable. An operable mode of indurating the raw pellets involvesheat-treating the latter to an elevated temperature near to but belowthe fusing point of the ore material. When thermally indurated underoptimum conditions, the pellets assume a mechanical strength equal orsubstantially equal 1 Claim. (Cl. 75-5) 2, to that of pieces (ofequivalent size) of the original ore material and hence may be shippedor .otherwise handled in mass without crushing or serious fracturing.

Drying out of the initially moist raw pellets, which is inevitable inany process for thermally indurating them, is attended by a markedreduction in mechanical strength. Upon loss of the moisture from betweenand on the particles constituting the pellets, the resulting dry, rawpellets become extremely fragile and frangible, exhibiting only aboutone-third to one-fifth the strength of the moist raw pellets: they fallapart when dropped a few times through a distance as little as an inchor two, and they scarcely can sustain a load of as much as a foot ofpellets carefully piled upon them.

When the temperature of the dry raw pellets is raised to about 400 F.,the pellets regain their original (moist state) mechanical, e. g.,compressive, strength; with further temperature rises they progressivelyincrease in strength, and when a temperature of the order of 780800 F.has been reached the pellets have acquired several (e. g., six or more)times their original mechanical strength and are, therefore, said tohave been incipiently hardened. Increasing the temperature above 780-800F. confers additional strength upon the incipiently hardened pellets upto the point of optimum induration. The temperature range between about200 and about 800 F. may properly be termed the sensitive range in theheat-treating of the pellets, and constitutes the interval during whichthe greatest care must be taken to relieve the frangible, 'fragilepellets from mechanical shock and from imposed pressure. This sensitiverange is of importance in any thermal induration process whereinas isthe case herethe moist raw iron oxide pellets are continuously orintermittently added to the upper surface of a continuously orintermittently descending column of iron oxide pellets, in greater orlesser degrees of induration, gravitationally descending incountercurrent to the flow of a. heating gas. In such case, a pelletmust have been carried through the sensitive range before there has beensuperimposed upon it a greater weightthan the pellet can sustain withoutfracture or deformation. l

A second diificulty is encountered in the heat hardening of thesepellets. If attempt is made to carry the pellets rapidly through thesensitive range at too high a rate of heating, the sudden release ofsteam derived from the initial moisture content causes the pellets toflyapart or spall.

Ihe maximum rate of heating which can be used safely withoutencountering serious spalling depends upon the diameter of the pelletsas well as upon their moisture content and the size dis-* tribution ofthe constituent ore particles. In actual practice, the pellets may varyin diameter from 4 inch to 1 /2 inches, with inch as a commonlyencountered size. With a inch pellet, for example, rates of heating ofseveral hundred degrees per minute up to 650 to 750 F. per minute may beemployed without encountering serious spalling, whereas with the largersized pellets a somewhat slower rate of heating should be adhered to.

Therefore, in carrying out the thermal induration by the general processreferred to above, the rate of heating must be carefully controlledbetween an upper and a lower limit: the Pellets, once they are dried,must be hurried through their sensitive range in order to avoid theircrushing under the progressively superimposed pressures, whereas therate of heating must be limited to prevent fission or spalling under thethermal shock.

According to the process of the present invention, this control of therate of heating through the sensitive range is effected by establishingand maintaining a thermally overbalanced operation in which the heatcapacity of the ascending stream of heating gas exceeds the heatcapacity of the pellets in the descending column thereof, during equaltimes, by a few (e. g., between about 2.5 and about percent andsufficient to effect the heating of a layer or layers of the initiallysubstantially unheated moist oxidic iron pellets through drying toincipient hardening in a zone, in said column, the lower boundary ofwhich zone is not more than about 18 inches below the stockline or uppersurface of said column. The overbalanced condition, and its desiredextent, will now be more specifically described, with reference to theaccompanying drawings, in which:

Fig. 1 is a graph showing temperature distribution in a column of ironoxide pellets being heat-treated under thermally balanced operation;

Figs. 2, 3 and 41 are graphs showing temperature distributions whenoperating at 2.5%, 5 and 10%, respectively, of thermal overbalance;

Fig. 5 is a graph showing temperature distribution when operating at 5%of thermal underbalance; and

Fig. 6 is a graph illustrating changes in mechanical strength asinitially moist iron oxide pellets are progressively heated from 60 toabout In the present process, the raw wet pellets are charged in a moreor less continuous fashion onto the upper surface or stockline of adescending column of pellets maintained within a suitable heatingchamber or shaft furnace. A continuous flow of a heating gas, e. g.,air, or air mixed with combustion products, is passed continuouslyupwardly through this column whereby to effect heating of the pellets ina conventional countercurrent manner. It is of importance to limit thetotal height of the column from the stockline measured downwardly to thelocus of the introduction of the heating gas to a minimum. In a furnacesuitable for commercial tonnages, which may rang in diameter from 10feet to feet, it is seldom practical to limit the height of the columnto a figure less than 10 feet for example. Satisfactory final mechanicalstrength of the heat hardened pellets can be attained at temperaturesranging from 1700 to 2300 F., i. e., near but not exceeding the fusionpoint of the ore material.

' If such a counter-current heating is undertaken in a column 16 feet indiameter and 10 feet tall, in which raw pellets are charged on thestoclzline at F. and the heating gas is introduced at 2000 F. at a plane10 feet below the stockline, maximum thermal efliciency will be realizedwhen the heat capacity of the ascending, heating gas per minute isexactly equal to the heat capacity of the pellets moving downwardly perminute. That is to say, if G represents the pounds of ascending gas perminute, and if Cp is the specific heat of the gas, the products G'XCpmay be termed F and represents the heat capacity per minute of theheating gas stream. Similarly, if S represents the pounds of solidspassing a given horizontal plane, and s represents the specific heat ofthe solids, the product S s may be designated as M and represents theheat capacity of the descending solids per minute. When F=M, the heatingoperation is termed theranally balanced. In the example at hand, withthe average diameter of the pellets inch, the solids are heated in auniform manner from stockline to the plane of gas influx at a rate ofl6.16 F. per inch and will attain a temperature of 1986 F. .10 feetbelow the stockline. In like manner, the ascending gas stream will becooled at the rate of- 16.16 F. per inch and will emerge from thestockline at 74 F. In this 10 ft. vertical column, the ga-s'at eachpoint will be 14 F. hotter than the pellets at the same point. Thisdifference between the gas temperature T(g) and the solid temperatureT(s) is the thermal head which is effective in transferring heat at theas-solid interface.

.If20,000 C. F. M. (cu. ft. per minute) of a diatomic heating gas isblown upwardly through the column and if 1450 G. T. (gross tons) of rawwet pellets per day is charged on the stockline (9.5 moisture--1320 G.T. per day of dried pe1- lets), the operation will be in thermalbalance, and the ratio of gas to dry solids will be 9.79 cu. ft. perpound or 8.86 cu. ft. per pound of wet pellets. Reference to Fig. 1indicates the temperature of distribution in this 10 ft. vertical columnin the ideal case in which the how both of gas and solids is uniformacross any horizontal plane and in which the loss of heat through thewalls of the furnace is negligibly small. Y r

It will be observed from Fig. 1 that under balanced operation thepellets attain the tel perature 212 F. at an elevation 9 inches belowthe stockline; It has been found that some 3 to 9 inches of verticalheight is required to remove the moisture from the pellets and thatsection of the vertical column lying between 9 inches and 14t0 17 inchesbelow the stockline may be termed the evaporative zone and is shown infine shading in Figure 1. In a zone from the 14 inch level down to aplane 45 inches below the stockline, the temperature of the pellets isless than 780 F. and in present language is said to be in the sensitivecondition. This region of me chanical feebleness or fragility is shownin dotted shading inFigure i. It is obvious that the pellets in thelayer lying 45 inches below the stockline have merely been restored tothe mechanical strength of the original wet pellets, are unable'tosupport the 45 inches of overlying pellets and impractical whenheat-hardening raw pellets of iron ore particles. It is necessary,therefore, to contract the extent of the sensitive region and to elevatethe 780 F. isotherm nearer the stockline. This is readily accomplishedby increasing the amount of heating gas forced upwardly through thecolumn while holding the rate of charging of pellets constant.

The thermal distribution in the column when 20,500 cu. ft. of gas isemployed is shown in Fig. 2. In this operation, the heating is said tobe overbalanced to the extent of 2.5 e. g., F is 2.5% greater than M,and the ratio of heating, gas to moist pellets is 9.09 cu. ft. per eachpound of the moist pellets. It is seen that the evaporative zone hasbeen brought nearer the stockline and diminished in vertical extent.Evaporation of moisture becomes rapid at a distance of only 3 inchesbelow the stockline and the pellets are essentially dry on reaching alevel 7 inches below the stockline. The sensitive zone in Figure 2extends from 9 inches to 16- inches below the stockline. The locationand extent of the evaporative zone and of the sensitive zone are shownas shaded regions in Figure 2. With carefully controlled size ofparticles and with a highly eiiicient rolling operation, the mechanicalstrength of the pellets is usually sufiic ient to permit an operationwith this temperature distribution in the pellet column.

In the more usual cases of practical operation, it is preferred that athermal over-balance of 5% be employed, which condition is attained byblowing 21,000 C. F. M. while charging 1450 G. T. of wet pellets perday; the ratio here is 9.96 C. F. per pound of dry pellets or 9.39 cu.ft. per pound of wet pellets. The temperature conditions obtaining inthe column with the 5% thermal overbalance are shown in Figure 3. Theevaporative zone extends from 1.5 inches to 3.5 inches below thestockline and the sensitive zone has been contracted to lie from 3.5 to8 inches below the stockline as is indicated in the shaded region. Thecrowding of the several isotherms into proximity of the stockline byemploying thermal overbalance in this manner and to this extent resultsin a temperature distribution which permits satisfactory treatment ofthe pellets without causing serious crushing or spalling.

It is noted that the thermal gradient near the stockline is 16.2? F. perinch in Figure l; is 41 per inch in Figure 2; and is 98 F. per inch inFigure 3. That is to say, by employing 2.5 to 5.0 percent over-balance,the normal 15 F. per inch temperature gradient is increased to 250 and600 percent respectively. In the three illustrated examples above, theaverage gradient in the upper half of the ore column is 2 1 F. per inchin Figure 2 and 282 F. per inch in Figure 3. The average thermalgradient in the lower half of the column is 8.4 per inch in Figure 2 and4.0 per inch in Figure 3. The effect of thermal overbalance is to bowthe temperature-distance curves upwardly making them concave when viewedfrom below in Figures 2 and 3. Equally well it can be stated thatthermal over-balance increases the average thermal gradient in the upperhalf of the charge column and simultaneously decreases the averagethermal gradient in the lower half of the column as compared with thestraight line temperature distribution shown in Figure 1.

The exact amount of over-balance required in any practical operationwill depend (1) on the heat capacity of the ascending gas stream whichell) 6 will depend upon the composition of the gas; (2) on the heatcapacity of the ore itself, including its moisture content, as well ason the state of oxidation of the iron content, 1. e., whether'hematiteor magnetite; (3) on the averagesize of the pellets and on thedistribution of sizes about the average; and 4) on the degree'ofuniformity of flow of (a) the gas ascending the colummand (b) thepellets descending the column. The effect of the variations in the abovelisted factors encountered in usual practice is seldom great and it isan unusual circumstance that the ratio of gas to solid will exceed therange of 8 to 14: cu. ft. per pound of pellets charged. i

The loss of thermal efficiency due to thermal over-balance is seldom adetermining criterion since the extent of over-balance required willusually lie between 2.5% and 10%. I The heat distribution has been foundto be extremelysensitive to the ratio of gas to solid. If the gas volumeis too low, the isotherms are distributed at too low a level below thestockline and collapse of the pellets in their mechanically feeblecondition results, causing clogging of the interstices between pelletsand serious channeling of the gas in its upward flow through the column.This irregular gas flow distorts the isotherms in the column from theirnormal loci as horizontal planes and the temperature conditions withinthe column become chaotic. On the other hand, if a thermal over-balancemuch in excess of 10% is employed, the rate of heating in theneighborhood of the stockline becomes excessive: and the moisture in theore is driven out at too rapid a rate, thus causingfracturing of thepellets during drying and the somewhat explosive spelling of thepellets.

When 22,000 C. F. M. of diatomic heating gas, per day, is blown upwardlyin the example used here as an illustration, the temperaturedistribution is as shown in Figure 4. In this operation about 9.75 cu.ft. of the heating gas are used per each pound of the moist pellets. Itis observed that the temperature gradient at the stockline is 400 F. perinch, that the pellets attain the temperature 2000 F. 16 inches belowthe stockline and that the evaporative zone and sensitive zone have beenbrought to within two inches of the stockline. In this operation thetemperature of the gas exhausting from the stockline is 450? F.,representing a substantial loss to the thermal efiiciency of theprocess. The rate of descent of the pellets through the column is almostexactly one inch per minute (over-all density of the wet pellets 127lbs. per cu. ft.). The rapid impingement of gas at 1250 F.thetemperature obtaining 3 inches below the stockline (of. Figure 4)- onthe four uppermost layers of inch pellets subjects these pellets todangerously rapid drying, and spalling of the pellets may beencountered. Successful operation can thus be realized by the controlleduse of thermal overbalance which will, in general, be confined insidethe critical range of 2.5% to 10%.

An important application of my invention concerns the indurating ofpelletsformed by rolling moist fines composed of concentrates producedfrom the magnetic concentration of iron bearing materials, particularlythe taconite' deposits occurring in the Lake Superior region.

In pelletizing magnetite fines, I maintain a continuously descendingcolumn of pellets within a vertical cylindrical refractory lined chamber24 feet inside diameter, with a vertical height between the blastentrance and the stockline of 1.6

aeoaasr 7 feet. The pellets analyze 65.2% Fe (dry basis) and contain10.3% H20 as charged. The wet over-all density is 127 lbs/cu. ft., andthe average size of the pellets is 0.55inch. I maintain through thisvertical column of pellets the upward flow of 67,000 cu. ft./min. ofheated gas flowing counter-current to the descending col umn of pellets.When the heated gas is pro duced by burning 171 lbs/min. of fuel oilwith 65,000 cu. ft./air, the entrance temperature of the hot gas intothe lower level of the column (blast entrance) is or may be about 2180F. and the composition of the gas entering the column is: 6.6% CO2; 7.3%H20; 10.4% 02; and 75.8% N2. The meanspecific heat of this gas mixturebetween 60 F. and 2180 F. is 0.0215 B. t. u.

per standard cubic foot per degree F. The mean specific heat of the drypellets, over the same temperature range, is 0.236 B. t. u. per poundper degree F. Were it not for the heat absorbed in the evaporation ofthe moisture and the heat generated from the oxidation of the FezOr,initially in the raw pellets, into FezOa thermal balance would beattained when I charge 6850 lbs/min. (44 G. T. per day) to produce 6150lbs/min. of dry pellets. The ratio of gas-to-ore here is 9.78'cu.ft./lb. of wet pellets or 10.83 lbs/min. of dry pellets.

In actual practice, it is, of course, not permissible to ignore the heatabsorbed and generated in the evaporation of moisture and the oxidationof magnetite, respectively, since the evaporation heat amounts to 17%and the oxidation heat to 22% of the total. In order to determine thethermal balance, it is desirable to construct a heat balance which readsas follows:

It is noted here that the over-all heat supply and heat requirements areequated in order to determine the calculated thermal balance of 67,000cu. ft./min. of heating gas with 7275 lbs/min. of wet pellets (6520lbs/min. dry pellets). The ratio here is 9.20 cu. ft./lb. of wet pelletsand 10.22 cu. ft./1b. of dry pellets.

In operation, three factors may tend to render a calculation such asthat above somewhat unreliable in carrying out my process: (1) thetemperature of distribution in the upper dozen inches of the chargecolumn is subject to severe temperature distortion from the curvesillustrated in the accompanying figures, due to the absorption of heatrequired to evaporate the water from the wet pellets, which causes localflattening of the temperature-distance diagram; (2) the generation of207 B. t. u./lb. of Fe in the oxidation of magnetite to ferric oxidecauses an upward bulge in-the temperature curve in the zone immediatelybelow the evaporation zone in the charge column, the heating effect ofthe ore oxidation being 20% greater than the cooling effect ofevaporation; and (3) the possible failure of the ascending gas stream toachieve uniformity of flow across any horizontal cross-section of thecharge column--a disturbing effect which is accentuated by failure ofthe charge column to exhibituniformity of downward mass flow across thesame horizontal sections of the column. It is all Welland good, from atheoretical standpoint, to adjustthe flow of gas and pellets to agreewith the thermal calculations and to exhibit a ratio of gas-to-pelletsat some predetermined figure. Actually, where the flow of pelletsdownwardly through certain horizontal sections of the column exceeds theaverage, and where (as likely as not) the upward flow of gasthrough'that area is less than the average, the ratio of gas-to-pelletswill diverge markedly from the overall average, with the result that toorapid heating of the raw pellets-in one area of the furnace section willcause objectionable spalling, due to rapid heating, while, concurrently,the raw pellets are heated too slowly in other areas in the samehorizontal section with frequent crumbling and disintegration of thepellets which have been subjected to too great compressive forces due totoo great a superposed burden of pellets in the column.

It has been found, therefore, that although the heat balance issuiiiciently accurate to determine the order. of magnitude of the flowof gas required for thermal balance per pound of pellets, it is not acompletely satisfactory criterion to guide the operator in controllingthe present process. It has been found, however, that observations ofthe temperature in horizontal sections several feet below the stocklineconstitute a very effective method of controlling the temperaturedistribution in the charge column. 7

In the present example, the temperature 8 feet below the stockline(mid-section of the column) will read 1120 F. when in thermal balance(average of inlet gas temperature 2180 F. and pellet temperature 60 F.)A few percent overbalance in the ore-pellet ratio will cause thetemperature at mid-section to rise several hundred degrees above 1l20IT, and the temperature observed at this mid-section will vary greatlyin response to minor, if not immeasurably small, variations in thegas-pellet ratio. Since the temperature observed at any cross-section islower in the furnace at the loci of the zones of ore oxidation andmoisture evaporation is greatly amplified as a result of smallvariations in the gas-pellet ratio, I have found that it serves as themost convenient and reliable method of controlling the necessaryoverbalance which is the basic feature of my present invention.

In any operating case, in practice it is necessary for the operator todetermine the optimum temperature of his probe thermocouple located ator near the mid-section of the column by observing the physicalcharacter of the indurated discharge from the column. This isreadilyascertained in any operating example of practice, but isdifficult to define in general terms due to the wide variation in thespecific heat of the pellets and gases employed, as well as the moresignificant deviations from uniformity of flow exhibited by the pelletsin their descent.

I claim:

The process of heat hardening pellets of magnetite fines which comprisesforcing a stream of an oxidizing gas mixture comprising air and gaseousproducts of combustion of a carbonaceous fuel, said oxidizing gasmixture being initially heated to a temperature between 1000 F: and thefusion temperature of the magnetite lines, upwardly in countercurrentheat exchanging contact with a continually descending column of thepellets initially containing about 10% moisture,

10 l and maintaining the heat capacity of the ascend- REFERENCES CITEDmg oxidizing gas mixture from t0 10% m The following references are ofrecord in the excess of the heat capacity 01 the descending me of thispatent: pellets by controlling the volume of the oxidizing gas mixtureused to between 9.09 and 9.75 cubic 5 UNITED STATES PATENTS feet pereach pound of the initially moist pellets Number Name Date whereby thepellets are heated to incipient in- 1,875,249 Mahler et a1 Aug. 30, 1932duration temperature by the time they have 2,131,006 Dean Sept. 20, 1938descended into said column a distance between 2,345,067 Osann Mar. 28,1944 $212: iflllllllzzznd not more than 16 inches from 10 OTHERREFERENCES 1 Proceedings of the Blast Furnace and Raw PERCY H. ROYSTER.Materials Committee, vol. 4 (1944), pages 54-58.

